This is what I have heard. “Monks, there are two people whom one cannot fully repay. Which two? One’s own mother and father. Having a life of a hundred years and living a hundred years, one might carry one’s mother on one shoulder and one’s father on the other, anointing, bathing, massaging, and shampooing them, even while they excreted and urinated there, but still one would not have done enough, one would not have repaid them. One might establish one’s mother and father as rulers of supreme authority over this whole earth with its abundance of seven treasures, but still one would not have done enough, one would not have repaid them. Why is this? Mothers and fathers do much for their children, bringing them up, feeding them, guiding them through this world.

“And yet, one who brings about the fullness of faith in parents who have little faith, who directs them towards and establishes them in the fullness of faith; one who brings about the fullness of virtue in parents who have little virtue, who directs them towards and establishes them in the fullness of virtue; one who brings about the fullness of generosity in parents who are stingy, who directs them towards and establishes them in the fullness of generosity; one who brings about the fullness of wisdom in parents who have little wisdom, who directs them towards and establishes them in the fullness of wisdom—by doing these things he has done enough, he has repaid them, he has done more than enough.”

From Sayings of the Buddha: A Selection of Suttas from the Pali Nikayas, translated by Rupert Gethin, Oxford: Oxford University Press, 2008, p. 251.